The relationship between global climate change and the emission of greenhouse gases, such as carbon dioxide (CO2), is well documented. Conventional fossil fuel combusting power plants, such as pulverized-coal-fired power plants, generate significant amounts of CO2. Therefore, there is a seemingly constant drive to improve the efficiency of such power plants and to develop improved technologies to reduce their CO2 emissions. One of the main solutions developed to achieve these goals is based on gasifying coal in a gasifier to generate syngas, which is combusted in a downstream combustor.
A recently published study by M. De Lallo, et al., “Evaluation of Innovative Fossil Cycles Incorporating CO2 Removal,” which was presented at the 2000 Gasification Technologies Conference in San Francisco, Calif., on Oct. 8-11, 2000, discusses several known processes for the removal/sequestration of CO2 from coal-fired plants. The study revealed that such CO2 removal and/or sequestration systems, when applied to the back end of a pulverized-coal-fired plant, can reduce plant efficiency by up to eleven percentage points with as much as a $30 per ton CO2 removal cost. For the purpose of comparison, all dollar amounts discussed herein are expressed in terms of 2003 U.S. dollars.
In conventional oxygen-blown Integrated Gasification Combined Cycle (IGCC) plants, for example, carbon monoxide (CO) is frequently water-gas shifted to hydrogen (H2) and CO2 upstream of a gas turbine. Such CO2 can then be separated and concentrated by absorption and stripping, or by membranes, and then compressed for sequestration. This type of a process, however, is energy intensive, costly, and lowers system efficiency due to CO2 regeneration from solvent and the energy losses associated with shifting. (Because the lower heating value (LHV) of hydrogen is less than that of CO, on a per mole basis, 15% of the LHV is lost when CO is shifted to H2. Therefore, more syngas needs to be generated from gasification in order to compensate for the shift loss. The low-grade heat from the shift reaction contributes to system losses.) It is estimated that there is an efficiency loss of six percentage points, combined with a CO2 removal cost of $15 per ton, for IGCC plants utilizing such processes.
U.S. Pat. No. 6,269,624 discusses a combined cycle power generation process for combusting gaseous fuel with oxygen, where a portion of the carbon dioxide in the exhaust gas of a gas turbine is recycled to the gas turbine combustor. Carbon dioxide accumulated in the recycled gas is condensed downstream from a gas compressor connected to the gas turbine.
U.S. Pat. No. 5,572,861 discloses an IGCC process utilizing recycled carbon dioxide as dilution fluid in a series of gas turbine combustors. Carbon dioxide exhausted from the gas turbines is compressed in a multistage compressor, including interstage separation of a portion of the carbon dioxide, to be conducted to a condensation unit. According to the patent, syngas is produced by gasifying coal in a pressurized gasifier with a mixture of oxygen and steam. This process utilizes very complicated equipment, such as a high temperature compressor for the syngas.
Great Britain Patent No. 1 298 434 describes a process in which coal is gasified by using pure oxygen, and the produced gas is burned in a boiler furnace with oxygen. Water vapor is condensed from the flue gas of the boiler furnace, and a portion of the remaining carbon dioxide is recycled back to the gasifier and the furnace to control temperatures in the gasifier and the furnace. Another portion of the carbon dioxide is compressed in a multistage compressor and cooled in several steps, so as to form liquid carbon dioxide. The thermal efficiency of this cycle is, however, not very high, since power is generated only by a low efficiency, steam turbine based, Rankine cycle.